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      07-01-2007, 08:03 AM   #25
Dr_Dirt
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Drives: E90 335i 6AT
Join Date: Apr 2007
Location: DFW

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Quote:
Originally Posted by Terry335 View Post
My autometer mechanical boost gauge. Stock I'd see ~8psi @ sea level, and at 4500' DA I'd see ~10psi. If what you said was correct re: gauges then the ECU is compensating even more than I thought.
Is the ECU compensating? More or less, however it depends on what you mean about the ECU compensating. The ECU sees the MAP sensor reading a negative pressure. For example where you are ~4500' your gauge should really read ~ -2psi, your gauge still reads zero because its diaphragm is equalized. However the MAP sensor is referencing an absolute vacuum or some other calibrated pressure therefore it knows the game, and sees that drop in pressure due to the higher altitude.
When you step on the gas the ECU spools the turbos until the MAP sensor reports that 8psi is achieved, but the ECU has to go the full range from -2psi-0-------8psi. Your boost gauge always starts from zero so it shows 0---------10psi.
It spools the turbos harder but the same boost level is achieved.

Last edited by Dr_Dirt; 07-01-2007 at 08:48 AM..
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